Mixing apparatus and method of operating same



I United States Patent 1111 3,545,728

[72] Inventor Richard Schultz, Uzwil, [50] Field ofSearch 259/46, 45,Switzerland 9,l0,25,26,l09, 110,169, 170, 178,161, 162, [2|] AppLNo.752,310 163 [22] Filed Aug. 13,1968 [45] Patented Dec-8, 1970Rel'erencesCnted [73] Assignee Gebruder Buhler AG UNITED STATES PATENTSGIILSWMM 2,145,428 1/1939 Motta 259/4 Priority a- 1967 2,544,202 3/1951Warner 259/169 1 w a d 2,596,672 5/1952 Gard 259/10 1 [3]] No. PrimaryExaminer-Robert W. Jenkins Attorney-McGlew and Toren ABSTRACT: A mixerfor the batch or charge mixing of plural [54] Mlxmc APPARATUS AND METHODOF components has at least one mixer shaft rotating in a mixer OPERATINGSAME trough and provided with mixing tools. The mixer shaft ischi'mannwmngs rotated at a first relatively low speed during mixing ofthe [52] U.S.CI 259/10, 1 components and, during at least the latterpart of the 259/46, 259/178 discharge of the mixture, is rotated at avery substantially [51] Int. B01l 7/02 higher speed. 1

PATENTEDDEE' 81970 3545728 SHEET 1 OF 3 Fig. 1

INVEN'I'UH Numm: SmvLTz WWW! PATENTEDDEC 8IQ7B 3.545728 saw 3 0F 3 IFig. 3

INVENTUR. \CHARD SCHULT WWI A16 75 -1 AN METHOD or OPERATING SAMEBACKGROUND OF THE INVENTION In a mixing process generallyknown as batchmixing, mix tures of different components are provided by a mixer. Thecomponents are charged into the mixer in a certain amount, mixed as abatch and discharged again as the final mixture. This batch mixing isefiect'ed by'a batch mixer,'and an extremely complete evacuation-of themixer is required particularly in view of variations of the compositionof the individual componentsto be mixed.jl )epending on the type ofmaterial components used for a particular mixture, the latter will havedifferent flow properties in discharge from the mixer.

Thus, it is known that both the 'so-called concentrated feed mixturesand various mixtures in the chemical industry frequently have poor. flowproperties. In addition, special socalled trace elements, which arepresent in the mixtures in a very fine distribution, play a differentrole in each mixture. Such trace elements must be'added in varyingamounts, de pending on the intended use. In particular, mixers in theconcentrated feed industries and in chemical industries must meet veryhigh requirements. regarding the residual discharge capacities, so thatone-"mixture is not impaired by undischarged residual deposits of apreceding mixture.

As a solution to the problem. of residual discharge, or dischargeresidue, it has already been suggested to feed a gas under pressurethrough thernixture shaft individually to nozzles arranged in the mixingtools, in order to blow out the mixer trough with-this gas underpressure and during the discharge of the mixer. This cleaning method hasthe advantage that the zones of the mixer trough within the range of thenozzles are well cleaned, but'it has the disadvantage that themixer-trough remains covered with a residue of the mixer material at allpoints or areas n'otcontactedby the current of gas under pressure,

SUMMARY or THE INVENTION This invention relates to the batch mixing ofseveralmaterials or components in a batchi'nixer and, more particularly,to an improved method of operating the batch mixer and to im- MIXINGAPPARATUS proved apparatus for operatingthe batch mixer.

In accordance with the invention the above-mentioned disadvantages with'respect'to complete evacuation of a batch mixer are avoided, inaccordance with the invention method, by driving the mixer shaftmountingthe mixing tools during the mixing process at a first speed perunit of time, and driving the shaft, during the discharge of the batchmixer, at a second speed per unit of time and which is higher than thefirst speed.

A batch mixer for mixing several components in a self-contained mixingprocess comprises, in accordance with the invention, at least one mixershaft rotating in 'a-mixer trough and provided with mixer tools,1the'mixer trough being provided with at least one selectively closabledischarge opening and the mixer shaft having f at" l east one drivingunit coupled thereto. The driving unithas selectively operable firstmeans for driving the mixer shaft ata first speed per unit of time, or afirst velocity, during the mixing process, and selectively operablesecond means for driving the" mixer shaft at a higher speed per unit oftime, or higher velocity, during discharge of the batch mixer. t

As a feature of the invention, first and second driving units areprovided as a means for driving the mixer shaft.

In accordance with another feature of the invention, an overrunningclutch is interposed between the first driving unit and the mixer shaft,with-a second driving unit being in direct driving connection with themixer shaft. In accordance with the invention, the driving units maycomprise various embodiments, and the inventioritalso includes specialprocedural or method steps, special designs of apparatus, and specialuses.

In addition, the present invention is'directed to the use of the mixerfor mixing primarily components ranging from granular to powdered andbatch charged into the mixer, and

also relates to the subsequent complete discharge of the mixture as abatch.

An object of the invention is to provide an improved method of operatinga batch mixer for mixing several different components or ingredients.

Another object of the invention is to provide improved batch mixingapparatus for the batch mixing of several components or ingredients.

A. further object of the invention is to provide a novel method andapparatus whereby a batch mixer may be completely evacuated at the endof a mixing operation.

Another object of the invention-is to provide such a method in which thetool carrying shaft of a batch mixer is driven at a first relatively lowangular velocity during the actual mixing operation and is driven at asubstantially higher angular velocity during at least the latter portionof the mixture discharge phase.

A further object of the invention is to provide a batch mixer includingdriving means operable to rotate a mixer shaft, carrying mixer tools, ata first relatively low angular velocity during the actual mixing of theingredients and at a second and much higher angular velocity during atleast the latter part of the mixture discharge phase.

For an understanding-of the principles of the invention, reference ismade to the following'description of typical embodiments thereof asillustrated in the accompanying drawings. i

. BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENTS Referring first to FIG. 1,.separate feeding lines 2, for therespective components or ingredients of a mixture to be batch mixed,lead into a batch balance 1. A hopper 3 is arranged beneath the balance1 and is associated with a batch mixer generally indicated at 10. Amixer trough 11, forming part of mixer 10,. has bearings 12 and 13 atopposite sides or ends thereof, and these bearings rotatably support amixer shaft 15. A pinion 16 is secured on one end of shaft 15, and adriving gear 17 is secured on the other end thereof. An overrunningclutch 20 is interposed between drive gear 17 and shaft 15.

Bearings 21 on trough-11 pivotally support a swingable bot tom closingflap'or door 25, through the medium of arms 22 secured to a shaft 26..To operate flap or door 25, shaft 26 is driven by a motor 29 through areduction gear 30. Beneath mixer 10, there is arranged a dischargetrough 40 provided with a discharge worm 43 driven by a motor 42, andtrough 40 has an outlet 45.

A first driving unit 50 is coupled with driving gear 17 and comprises adriving motor 51 driving a pinion 52 meshing with gear 17. A seconddriving unit ,60 is coupled with pinion 16, and comprises a motor 61driving a gear 62 meshing with pinion 16. All the mentioned .devices aresupported in a known manner on foundations, similar to therepresentation of the supports for the driving motors 51 and 61.

While FIG. 1 shows the essential discharge elements of the mixer, FIG. 2shows the driving system actually used. As a variant to the two drivingmeans shown in FIG. 1, V-belt drives preferably are used in theembodiment of the invention shown in FIG. 2. For this purpose, motor 51has a drive pulley 54 which drives a driven pulley 57 connected by anoverrunning clutch 20 to shaft 15, the two pulleys being interconnectedby V-belts 55. In a similar manner, driving motor 61 has a drivingpulley 64 connected by a V-belt 65 with a V-belt pulley 66 secured onmixer shaft 15.

Conductors 72 extend from a source of a.c. potential 70 to a main switch82, and conductors 74 connect switch 82 to driving motor 51. A secondswitch 84 is provided, being interposed between conductors 74 andconductors 76 connected to driving motor 61.

The components to be mixed preferably arrive at the batch mixer afterhaving been dosed or weighted. In the majority of cases, the mixer shaft15 carrying the mixing tools can be driven, even during filling of thebatch mixer 10, at a first speed per unit of time or first angularvelocity. This first velocity thus serves for carrying out the mixingprocess, which later requires a certain length of time. After the mixingprocess has been completed, discharge of batch mixer is initiated,generally by opening the bottom flap 25. However, since discharge withconventional means is not satisfactory, in that it leaves a residue ofmaterial in the mixer, mixer shaft is driven, during the dischargeperiod, at a second speed per unit of time or second velocity which ishigher than the first velocity.

There are two possible arrangements. A single driving unit can be sodesigned that it can be switched from the first velocity to the secondvelocity, or the mixer shaft 15 can be connected by an overrunningclutch with the driving unit for the first velocity and a second drivingunit can be provided for the second and higher velocity, the seconddriving unit being in direct driving connection with mixer shaft 15.Overrunning clutch works in a manner similar to a freewheel drive,wherein it is possible to have; on one side the clutch, a higher speedwithout the speed being influenced by a different speed being applied tothe other side of the clutch. Such overrunning clutches are generallyknown and are commercially available. After completion of discharge, thesecond higher velocity drive is disconnected or deenergized. Thereupon,it tends to decelerate and, after a certain time, is carried along bythe first drive having the lower or smaller velocity. The dischargedmaterial is fed to another processing state.

The apparatus represented in FIGS. 1 and 2 is suitable for performingthe method of the invention. Upon closing the main switch 82, drivingmotor 51 is energized from polyphase a.c. source 70 through multipleconductors 72 and 74. Through pulley 54, belts 55 and pulley 57 motor 51drives the mixer shaft 15 through overrunning clutch 20, in thearrangement of FIG. 2. In the arrangement of FIG. 1, the shaft 15,carrying the mixing tools 19 is driven, through overrunning clutch 20,but motor 51 through pinion 52 and driving gear 17.

The components to be mixed arrive in balance 1 through the feed elements2, such as the down pipes 2 shown in FIG. 1. From balance 1, the feedelements are fed through feed hopper 3 to the batch mixer 10 for theself-contained mixing process. The desired mixing of the individualcomponents or ingredients is effected while mixer shaft 15, carryingmixing tools 19, is driven at the first and lower angular velocity.After completion of the mixing process, discharge motor 29 is energizedby switching means which have not been shown. Through reduction gear 30,motor 29 rotates shaft 26 to open discharge flap or door connected toshaft 26 through the arms 22. The evacuation of the batch mixer is thusinitiated and, during the discharge of the mixture, shaft 25 is rotatedat the second and higher angular velocity by energizing driving motor 61to drive pinion 16 and shaft 15 through driving gear 62, in thearrangement of FIG. 1. In arrangement of FIG. 2, driving motor 61 isenergized by closure of switch 84, and through multiple conductors 76,to rotate driving pulley 64 and thus to rotate driven pulley 66 andshaft 15 through the medium of belt 65. The rotation of shaft 15,carrying the mixing tools 19, at the second and much higher angularvelocity insures a substantially complete evacuation of the mixer 10.During driving of the shaft 15 by motor 61, the overrunning clutch 20permits the gear 17 to be continuously driven without effecting therotation of shaft 15 by motor 61. After completion of the discharge,motor 29 is operated to rotate shaft 26, mounted in bearings 21, toclose flap or door 25 through the medium of arms 22, and switch 84 isopened so that, after a certain time, the angular velocity of mixershaft 13, carrying mixing tools 19, is reduced to the first and lowerangular velocity corresponding to operation by the motor 51 As best seenin FIG. 1, the mixedmaterial or mixture arrives in collecting trough 40having a discharge worm 43 driven by a motor 42. Worm 43 moves thematerial along trough 40 to which the discharged outlet 45,- from thedischarge material is fed to another processing stage (storage,pressing, bag filling, etc). I

The substantially improved residual discharge resulting from theinvention method of operating the batch mixer is based on the fact thatdue to the much higher speed of the mixing tools during discharge of themixture, air eddies break off on the trailing sides thereof, and theseeffect a detachment of the mixture particles adhering to the surfaces ofthe mixer. However, since batch mixers in many cases have mixing toolscovering practically without interruption the entire length of the mixertrough, this method of residual discharge acts on all of the mixer wallzones. It will be understood that the higher discharge speed of themixer shaft 15 can be started at any time during evacuation of the mixertrough, either at the start of the opening of the discharge flap or?door 25 or a certain time following such opening.

The arrangement shown in FIGS. land 2 represent a simple design ofapparatus for performing the method of the invention. These arrangementsare particularly of interest in all new construction, since the entirecontrol expenditure is very low and, by virtue of the use of theoverrunning clutch 20, the provision of various locking circuitsisobviated. By contrast, in certain alterations or in order to meetspecial operating requirements, it may be desirable to provide only onedriving motor for the mixer and to operate this motor by aspecialcontrol. An arrangement of this type is shown in FIG. 3.

Referring to FIG. 3, the batch mixer 10 is associated with a feed hopper3 and comprises the mixer trough 11 havingthe two bearings 12 and 13rotatably supporting the mixer shaft 15 having the driving pulley 57thereon. A drive unit 150 is coupled with shaft driving pulley 57, andcomprises a commutator motor 151 having an output pulley 54 connected todriving pulley 57 by V-belts 55. In this case, the closing flap or doorof batch mixer 10 is provided with a'hydraulic system comprising a pump101 connected by a pressure line 102 to a reversing valve 103. Controllines 105 and 106 connect valve 103 to hydraulic cylinder 111, and atank line or return line 108 connects the valve 103 to a tank 110containing the hydraulic medium 109. The inlet of pump 101 communicateswith tank 109.

Hydraulic cylinder 111 is supported on trough 11" at 114, and a pistonrod 112, connected with a piston (not shown) in cylinder 111, isconnected with flap or door 25 by a hinge or pivoting connection 116. Acontrol switch is mounted on mixer 10 and is a reversing switch havingcontacts 122. In the open position of flap or door 25, switch 122 isreversed by a control plate 118 on door or flap 25. One contact of theworking contacts 122 is connected by a line with the grounded side of apolyphase a.c. potential source 70, while the other contact of theworking pair is connected by a line to a timing element 136. A conductor137 connects timing element 136 to one end of the operating coil 138 ofa reversing switch 140, and a line 131 connects the opposite end ofwinding 138 to one of the output conductors of source 70.

Three conductors 72 connect source 70 to a control device 160 comprisingcontrollable rectifiers. Commutator motor 151 of driving unit isconnected by three conductors 74' to control device 160. In addition tooperating coil 138, a first speed setting device 141 and a second speedsetting device 142 are connected to lines 130 and 131. First speedsetting device 141 controls the angular velocity of shaft 15 during themixing process, and the second speed setting device 142 provides thesecond or higher angular velocity for shaft 15 during discharge of thebatch mixer. Devices 141 and 142 are connected by respective conductors146 and 147 to respective contacts of reversing switch 140, andconductors 148 connect reverseswitch 140 to control device 160.

The apparatus illustrated in FIG. 3, for performing the method of theinvention, worksin the following manner. In the illustrated position ofall control elements, the batch mixer is set for mixing process.Independence on the setting of the speed setting device 141,.for'thebias control of the controllable rectifiers and controldevice 160,commutator motor 151 of driving unit 150 runs at a-certain firstrelatively low speed. Motor 151 thus drives mixer'shaft 15 at a firstrelatively low angular velocity, through the medium of pulley 54,V-belts 55, and driving pulley 57. The motor'speed can be set by thephase gating control, which, is "a known procedure. The generation ofthe control pulses necessary for the three phases is effected, for eachspeed, in the respective setting devices 141m 142.

After the ingredientshavebeen thoroughly mixed, the entire mixed batchis dischargedeThis is effected by. switching the reversing valve 103.Responsive to such switching'of valve 103, pistonrod 112 is moved"upwardly by the piston in hydraulic cylinder 111,Iflap .or dischargegate 25 is opened and, in the final open po'sitionfthereof, controlswitch 120 is actuated by control plate 118. This results in exitationof operating coil 138, with a predetermined time delay, through timingelement 136. When coil 138 is energized, reversing switch 140 isreversed, disconnecting speed setting device 141 from control device 160and connecting speed setting device 142 to control device 160. Speedsetting device 142 is set to operate control device 1 60 to provide asubstantially higher second operating speed to commutator motor 151,energized from polyphase a.c. source 70. Motor 151 thus accelerates tothe higher second speed. As a result, the mixer shaft is rotated at asubstantially higher angular velocity and effects a better residualdischarge. f i I After the discharge, -,'hydraulic reversing valve 103is reversed again to the position illustrated in the drawing. Piston'rod 112 is moved downwardly relative to hydrauliccylinder 111 to closeflap or dischargedoor 25 of batch mixer 10. Control switch 120, beingtherebydisengaged by control plate 118, opens and disconnects operatingcoil 138 from the voltage source. Thus, reversing switch 140 is reversedto the position illustrated in the drawing to reconnect speed settingdevice 141 with control device 160 and to disconnect speed settingdevicel42from control device 60. Control device 60 is thus set for theinitial relatively low first speed of motor 51, and the motor 51'decelerates' to, this speed. Batch mixer 10 can be refilled withmaterial or ingredients to be mixed, through'the hopper 3, sothat. anewmixing operation is initiated.

The machine of FIG. -3. is-characterized in that both the lower speedrange, for the mixing process, and the higher speed range, for theresidual discharge, can be readily adapted to the-properties of thematerials. Existing mixers can be easily so changed that the new methodfor operating the mixer can be readily applied.

Other arrangements for carrying out the method, particularly withrespect to the driving of the mixer, are readily possible within thescope of the invention. Thus, hydraulic or pneumatic driving units maybe of advantage in certain cases, since with these it isonly necessaryto adjust pressure setting valves or hydraulic volume governors in thefeed line to the driving element.

Mixers for performing the method of the invention are particularlysuitable for the preparation of mixtures of individual components inbatches for the production of feeds, foods, etc., as well as for theproduction of chemical inorganic or organic mixtures of bases,semifinished products, or end products. The complete evacuation of thebatch'mixer for mixing different components which are enriched withtrace elements, frequently in extreme mixing ratios of 1:1000or I: 10000(trace element to remaining mixturecomponents) is of major importanceand is made possible by the invention method as well as the apparatusfor performing this method.

' ble to mix plural mixture components in a self-contained mix- I claim:l. A method of operating a batch mixer, of the type operaing phase andthen to discharge the mixture, and of the type having at least onerotatable mixer shaft carrying mixirig tools rotatable in a mixertrough, said method comprising, during mixing of the components to forma mixture, rotating the mixture shaft at a first preselected angularvelocity; discharging the mixture simultaneously along substantially theentire length of the trough; and, during discharge of the mixture,rotating the mixer shaft at a second preselected angular velocity whichis substantially higher than the first preselected angular velocity andsufliciently high to generate air eddies'at the trailing sides of themixing tools to air sweep substantially the entire inner surface of thetrough.

2. A method of operating a batch mixer, of the type operable to mixplural mixture components in a self-contained mixthe first preselectedangular velocity; the'mixer shaft being rotated at said secondpreselected angular velocity after the mixer has been half evacuated.

3. A method of operatinga batch mixer, of the type operable to mixplural mixture components in a self-contained mixing phase and then todischarge the mixture, and of the type having at least onerotatablemixer shaft carrying mixing tools rotatable in a mixer trough,said method comprising, during mixing of the components to formamixture, rotating the mixture shaft at a first preselected angularvelocity; and, during discharge of the mixture, rotatingthe mixer shaftat a second preselected angular velocity which is substantially higherthan the first preselected angular velocity; and including the steps of,during the first part of the discharge of the mixture, rotating themixer shaft at the first preselectedangular velocity; and thereaftercompleting the discharge while rotating the mixer shaft at the secondpreselected angular velocity.

4. In a batch mixer of the type operable to mix plural mixturecomponents in a self-contained mixing phase and then to discharge themixture, and having at least one rotatable mixer shaft carrying mixingtools rotatable in a mixer trough having at least one discharge openingand means to close each opening, and having driving means for the mixershaft, the crosssectional area swept by the mixing tools being only veryslightly less than the cross sectional area of the trough interior, theimprovement comprising, in combination, selectively operable firstmeans, in said driving means, operable to rotate said mixer shaft at afirst angular velocity during mixing of the components to form amixture; and selectively operable second means in said driving means,operable to rotate said mixer shaft at a second and substantially higherangular velocity during discharge of the mixture and sufficiently highto generate air eddies at the railing sides of the mixing tools to airsweep substantially the entire inner surface of the trough.

5. In a batch mixer, the improvement claimed in claim 4, in which saidselectively operable first means comprises a first driving unitoperatively associated with said mixer shaft; said selectively operablesecond means comprising a second unit operatively associated with saidmixer shaft.

6. In a batch mixer, the improvement claimed in claim 4, in which saidselectively operable first and second means comprises an ac. drivingmotor; and an infinitely variable electronic control connecting saida.c. driving motor to a source of a.c. potential.

7. In a batch mixer of the typeoperable to mix plural mixture componentsin a self-contained mixing phase and then to discharge the mixture, andhaving at least one rotatable mixer shaft carrying mixing toolsrotatable in a mixer trough having at least one discharge opening andmeans to close each opening, and having driving means for the mixershaft, the improvement comprising, in combination, selectively operablefirst means, in said driving means, operable to rotate said mixer shaftat a first angular velocity during mixing of the components to form amixture; and selectively operable second means in said driving means,operable to rotate said mixer shaft at a second and substantially higherangular velocity during discharge of the mixture; said selectivelyoperable first means comprising a first driving unit operativelyassociated with said mixer shaft; said selectively operable second meanscomprising a second unit operatively associated with said mixer shaft;and an overrunning clutch interposed between said first driving unit andsaid mixer shaft; said second driving unit being in direct drivingconnection with said mixer shaft.

8. In a batch mixer, the improvement claimed in claim 7, in which saidfirst driving unit remains connected to said mixer shaft during rotationof said mixer shaft by said second driving unit at said secondpreselected angular velocity and is overrun by said second driving unitthrough said overrunning clutch.

9. In a batch mixer of the type operable to mix plural mixturecomponents in a self-contained mixing phase and then to discharge themixutre, and having at least one rotatable mixer,

shaft carrying mixing tools rotatable in a mixer trough having at leastone discharge opening and means to close each opening, and havingdriving means for the mixer shaft, the improvement comprising, incombination, selectively operable first means, in said driving means,operable to rotate said mixer shaft at a first angular velocity duringmixing of the components to form a mixture; and selectively operablesecond means in said driving means, operable to rotate said mixer shaftat a second and substantially higher angular velocity during dischargeof the mixture; said mixer trough being a horizontally oriented oblongtrough having said mixer shaft rotatably mounted therein and providedwith an upper feed opening and said discharge opening at the lowerportion; said selectively operable first means comprising a firstdriving unit; and an overrunning clutch connecting said first drivingunit to one end of said mixer shaft; said second selectively operablemeans comprising a second driving unit in direct connection with theopposite end of said mixer shaft.

10. in a batch mixer of the type operable to mix plural mixturecomponents in a self-contained mixing phase and then to discharge themixture, and having at least one rotatable mixer shaft carrying mixingtools rotatable in a mixer trough having at least one discharge openingand means to close each opening, and having driving-means for the mixershaft, the improvement comprising, in combination, selectively operablefirst means, in said driving means, operable to rotate said mixer shaftat a first angular velocity during mixing of the components to form amixture; and selectively operable second means in said driving means,operable to rotate said mixer shaft at a second and substantially higherangular velocity during discharge of the mixture; said selectivelyoperable first and second means comprising a changeable pole dn'vingunit and a pole changing control operatively associated with saidchangeable pole driving unit.

11. In a batch mixer of the type operable to mix plural mixturecomponents in a self-contained'mixing phase and to discharge themixture, and having at least one rotatable mixer shaft carrying mixingtools rotatable in a mixer trough having at least one discharge openingand means to close each opening, and having driving meansfor the mixershaft, the improvement comprising, in combination, selectively operablefirst means, in said driving means, operable to rotate said mixer shaftat a first angular velocity during mixing of the components to form amixture; and selectively operable second means in said driving means,operable to rotate said mixer shaft at a second and substantially higherangular velocity during discharge of the mixture; said selectivelyoperable first and second means comprising a pressure medium operateddriving unit operatively connected to said mixer shaft; and a pressuremedium control means operatively associated with said pressure mediumdriving unit.

12. In a batch mixer of the type operab e to mix plural mixturecomponents in a self-contained mixing phase and then to discharge themixture, and having at least one rotatable mixer shaft carrying mixingtools rotatable in a mixer trough having at least one discharge openingand means to close each opening, and having driving means for the mixershaft, the improvement comprising, in combination, selectively operablefirst means, in said driving means, operable to rotate said mixer shaftat a first angular velocity during mixing of the components to form amixture; and selectively operable second means in said driving means,operable to rotate said mixer shaft at a second and substantially higherangular velocity during discharge of the mixture; said selectivelyoperable first means comprising a first driving unit operativelyassociated with said mixer shaft; said selectively operable second meanscomprising a second unit operatively associated with said mixer shaft;said second driving unit rotating idly during rotation of said mixershaft by said first driving unit at said first preselected angularvelocity.

